Title: Sponge: Draw the four types of reflectors.
1Sponge Draw the four types of reflectors.
2Light from different directions focuses at
different points, and an image is formed near the
prime focus.
3Light entering at great angles cannot be
accurately focused. Off-center star images
acquire tails (coma).
4Coma worsens as the picture moves farther from
the center of view.
5A Schmidt telescope uses a correcting lens, which
sharpens the final image of the entire field of
view.
6This image is focused on a specially shaped piece
of photographic film (often called a Schmidt
Camera)
7Light can also be focused on a photometer. This
measures the total light received on all or part
of the image.
8This light information can be split into its
spectrum on a photographic plate or be stored
on a computer.
9A larger telescope can gather more light, so it
is said to have a large collecting area.
10Angular resolution is the ability to distinguish
between two adjacent objects in the sky (with or
without a telescope).
11When the angular resolution is greater than the
separation of the objects, all that can be seen
is a fuzzy blob.
12Even large telescopes have angular resolution
problems because of atmospheric blurring.
13Atmospheric blurring is the result of the
different densities of the air deflecting the
incoming light slightly.
14This causes the image to move around on the
detector (or on our retina).
15This continual movement of the image causes stars
to twinkle.
16After a few minutes of exposure time, the image
has been smeared out over a circular region. This
is called the seeing disk.
17The Hubble Space Telescope avoids these problems
by placing the telescope above the atmosphere.
It was launched into Earth orbit in 1990.
18Hubble gives images that are up to 20 times
sharper than those of ground-based telescopes.
19Most newer telescopes use charge-coupled devices
(CCDs) rather than photographic film.
20The tiny picture elements on CCDs are called
pixels.
21CCDs have great benefits
- 1) gather more light,
- 2) gather light info faster,
- 3) images can be stored digitally,
- 4) noise can be eliminated.
22Radio telescopes detect cosmic radio waves.They
can be used 24 hours a day and weather has no
effect on the waves.
23Radio astronomy opens up a new picture of the
universe. Objects that produce a great deal of
visible light are not necessarily very high in
radio wave emission.
24Objects that emit detectable levels of radio
waves may be completely undetectable in the
visible region of the spectrum.
25Emissions can also be detected in the other
regions of the spectrum. Ultraviolet, infrared,
microwaves, gamma rays, x-rays, etc.
26Sponge Why do stars twinkle?Why dont
planets twinkle?
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